Metals from Groups I and II of the periodic table are commonly used to initiate the polymerization of monomers into polymers. For example, lithium, barium, magnesium, sodium, and potassium are metals that are frequently utilized in such polymerizations. Initiator systems of this type are of commercial importance because they can be used to produce stereo regulated polymers. For instance, lithium initiators can be utilized to initiate the anionic polymerization of isoprene into cis-1,4-polyisoprene or to initiate the polymerization of 1,3-butadiene into high vinyl polybutadiene.
The polymers formed in such polymerizations are terminated with the metal used to initiate the polymerization and are sometimes referred to as living polymers. They are referred to as living polymers because the polymer chains which are terminated with the metal initiator continue to grow or live until all of the available monomer is exhausted. Polymers that are prepared by utilizing such metal initiators normally have structures which are essentially linear and normally do not contain appreciable amounts of branching.
Such polymers are normally compounded with sulfur, carbon black, accelerators, antidegradants and other desired rubber chemicals and are then subsequently vulcanized or cured into the form of a useful article. It has been established that the physical properties of such cured rubbers depend upon the degree to which the carbon black is homogeneously dispersed throughout the polydiene rubber. This is in turn related to the level of affinity that carbon black has for the particular polydiene. This can be of practical importance in improving the physical characteristics of rubber articles which are made utilizing polydienes. For example, the rolling resistance and traction characteristics of tires can be improved by improving the affinity of carbon black to the polydiene rubbers utilized therein. Therefore, it would be highly desirable to improve the affinity of a given polydiene rubber for carbon black. This is because a better dispersion of carbon black throughout polydiene rubbers which are utilized in compounding tire tread compositions results in a lower hysteresis value and consequently tires made therefrom have lower rolling resistance. Low tan delta values at 60.degree. C. are indicative of low hysteresis and consequently tires made utilizing such polydienes with low tan delta values at 60.degree. C. have lower rolling resistance. Better dispersions of carbon black throughout the polydiene rubber also cause higher tan delta values at 0.degree. C. which causes tires made therefrom to have better traction characteristics.